COLLOIDAL DISPERSIONS Dispersed systems consist of particulate matter (known as the dispersed phase), distributed throughout a continuous phase (known as dispersion medium). CLASSIFICATION OF DISPERSED SYSTEMS On the basis of mean particle diameter of the dispersed material, three types of dispersed systems are generally considered: a) Molecular dispersions b) Colloidal dispersions, and c) Coarse dispersions Molecular dispersions are the true solutions of a solute phase in a solvent. The solute is in the form of separate molecules homogeneously distributed throughout the solvent. Example: aqueous solution of salts, glucose Colloidal dispersions are micro-heterogeneous dispersed systems. The dispersed phases cannot be separated under gravity or centrifugal or other forces. The particles do not mix or settle down. Example: aqueous dispersion of natural polymer, colloidal silver sols, jelly Coarse dispersions are heterogeneous dispersed systems in which the dispersed phase particles are larger than 0.5µm. The concentration of dispersed phase may exceed 20%. Example: pharmaceutical emulsions and suspensions COMPARISON OF CHARACTERISTICS THREE DISPERSED SYSTEMS Molecular dispersions Colloidal dispersions Coarse dispersions 1. Particle size <1 nm 1 nm to 0.5 µm >0.5 µm 2. Appearance Clear, transparent Opalescent Frequently opaque 3. Visibility Invisible in electron Visible in electron Visible under optical microscope microscope microscope or naked eye 4. Separation Pass through semipermeable Pass through filter paper but Do not pass through normal membrane, filter paper do not pass through filter paper and semipermeable membrane semipermeable membrane 5. Diffusion Undergo rapid diffusion Diffuse very slowly Do not diffuse 6. Sedimentation No question of settling Do not settle down Fast sedimentation of dispersed phase by gravity or other forces SHAPE OF COLLOIDAL PARTICLES The shape adopted by colloidal particles in dispersion is important because the more extended the particle, the greater is its specific surface and the greater is the opportunity for attractive forces to develop between the particles of the dispersed phase and the dispersion medium. In a friendly environment, a colloidal particle unrolls and exposes maximum surface area. Under adverse conditions, it rolls up and reduces its exposed area. 1 The shapes that can be assumed by colloidal particles are: (a) spheres and globules, (b) short rods and prolate ellipsoids (rugby ball-shaped/elongated), (c) oblate ellipsoids (discus-shaped/flattened) and flakes, (d) long rods and threads, (e) loosely coiled threads, and (f) branched threads The following properties are affected by changes in the shape of colloidal particles: a) Flowability b) Sedimentation c) Osmotic pressure d) Pharmacological action. TYPES OF COLLOIDAL SYSTEMS Based on the interaction between dispersed phase and dispersion medium, colloidal systems are classified as (a) Lyophilic colloids (solvent-loving) (When the dispersion medium is water, it is called hydrophilic colloids and if the dispersion medium is an organic solvent, it is called hydrophobic colloids) (b) Lyophobic colloids (solvent-hating) Difference between Lyophilic colloids and Lyophobic colloids Lyophilic colloids Lyophobic colloids Colloidal particles have greater affinity for the Colloidal particles have little affinity dispersion medium for the dispersion medium Owing to their affinity for the dispersion medium, Material does not disperse spontaneously, the molecules disperse spontaneously to form and hence lyophobic sols are prepared colloidal solution by dispersion or condensation methods These colloids form “reversible sols” These colloids form “irreversible sols” Viscosity of the dispersion medium is increased Viscosity of the dispersion medium is greatly by the presence of the lyophobic colloidal not greatly increased by the presence of particles lyophilic colloidal particles Dispersions are stable generally in the presence of Lyophobic dispersions are unstable in the electrolytes; they may be salted out by high presence of even small concentrations concentrations of very soluble electrolytes of electrolytes Dispersed phase consists generally of large organic Dispersed phase ordinarily consists molecules such as gelatin, acacia lying within of inorganic particles, such as gold or colloidal size range silver Preparation of Lyophilic Colloids This simple dispersion of lyophilic material in a solvent leads to the formation of lyophilic colloids. Preparation of Lyophobic Colloids The lyophobic colloids may prepared by 2 (a) Dispersion method (a) Condensation method Dispersion methods: This method involves the breakdown of larger particles into particles of colloidal dimensions. The breakdown of coarse material may be effected by the use of the Colloid mills, Ultrasonic treatment in presence of stabilizing agent such as a surface active agent. Condensation method: In this method, the colloidal particles are formed by the aggregation of smaller particles such as molecules. These involve a high degree of initial supersaturation followed by the formation and growth of nuclei. Supersaturation can be brought about by (i) Change in solvent: For example, if sulfur is dissolved in alcohol and the concentrated solution is then poured into an excess of water, many small nuclei form in the supersaturated solution. These grow rapidly to form a colloidal sol. If a saturated solution of sulphur in acetone is poured slowly into hot water the acetone vaporizes, leaving a colloidal dispersion of sulphur. (ii) Chemical reaction: For example, colloidal silver iodide may be obtained by reacting together dilute solutions of silver nitrate and potassium iodide. If a solution of ferric chloride is boiled with an excess of water produces red sol of hydrated ferric oxide by hydrolysis. Purification of Colloids When a colloidal solution is prepared, it often contains certain electrolytes which tend to destabilize it. The following methods are used for purification of colloids: (a) Dialysis: At equilibrium, the colloidal material is retained in compartment A, whereas the sub-colloidal material is distributed equally on both sides of the membrane. By continually removing the liquid in compartment-B, it is possible to obtain colloidal material in compartment-A which is free from sub-colloidal contaminants. The process of dialysis may be hastened by stirring, so as to maintain a high concentration gradient of diffusible molecules across the membrane and by renewing the outer liquid from time to time. [Open circles: colloidal particles, solid dots: electrolyte particles] (b) Ultrafiltration Colloidal dispersion can pass through an ordinary filter, because the pore size of the filter is large. If this filter paper is impregnated with collodion (syrupy solution of nitrocellulose), the 3 pore size reduces. Such modified filter papers are called ultrafilters. By applying pressure (or suction) the solvent and small particles may be forced across a membrane but the larger colloidal particles are retained. This process is referred to as ultrafiltration. (c) Electrodialysis In the dialysis unit, the movement of ions across the membrane can be speeded up by applying an electric current through electrodes induced in solution. The electric potential increases the rate of movement of ionic impurities through a dialysing membrane and so provide a more rapid means of purification. The dialysis membrane allows small particles (ions) to pass through but the colloidal size particles (hemoglobin) do not pass through the membrane. Association/Amphiphilic Colloids Surface active agents have two distinct regions of opposing solution affinities within the same molecule or ion and are known as amphiphiles. When present in a liquid medium at low con- centrations, the amphiphiles exist separately and are of subcolloidal size. As the concentration is increased, aggregation occurs over a narrow concentration range. These aggregates, which may contain 50 or more monomers, are called micelles. Because the diameter of each micelle is of the order of 50Å, micelles lie within the colloidal size range. The concentration of monomer at which micelles are formed is termed as critical micelle concentration (CMC). The number of monomers that aggregate to form a micelle is known as the aggregation number of the micelle. In water, the hydrocarbon chains of amphiphiles face inward into the micelle to form their own hydrocarbon environment. Surrounding this hydrocarbon core are the polar portions of the amphiphiles associated with the water molecules of the continuous phase. 4 Figure: Spherical micelle of an anionic association colloid in aqueous media The association colloid can be classified as anionic, cationic, nonionic, or ampholytic (zwitterionic) depending upon the charges on the amphiphiles. The opposite ions bound to the surface of charged micelles are termed counter ions or gegenions, which reduces the overall charge on the micelles. The viscosity of the system increases as the concentration of the aniphiphile increases because micelles increase in number and become asymmetric. In aqueous solutions, the CMC is reduced by the addition of electrolytes, salting out may occur at higher salt concentrations. OPTICAL PROPERTIES OF COLLOIDS Tyndall effect When a strong beam of light is passed perpendicularly through two solutions (1) true solution and (2) colloidal solution place against a dark background: 1. The path of light beam is not visible in case of true solution. 2. The path of light beam is